Apparatus and method for halftone hybrid screen generation

ABSTRACT

The present invention is a method and apparatus allowing the application of two or more distinct halftone types in the rendering of a single image. The invention employs a halftone selector of a threshold type, or as an alternative a segmenter type. The halftone selector determines which areas of an image will receive which type of grayscale halftone treatment on a pixel by pixel basis. Examples of various halftone types include stochastic, clustered dot, line screen, and other high addressability types. Each pixel of data is treated by the appropriate halftoner circuit which in response outputs digital data. A controller circuit remaps the digital data into the appropriate width and position signals for a pulse width position modulator, including any other signals as needed for any additional inverter circuitry. The pulse width position modulator generates a video signal which may be inverted by the inverter circuit as responsive to the controller circuit remapping determination. This final video signal is provided to an image output terminal.

BACKGROUND OF THE INVENTION AND MATERIAL DISCLOSURE STATEMENT

The present invention relates to processing an image signal utilizingpulse width position modulation techniques in combination with multiplehalftone processes to improve that image for reproduction.

Many printing devices are not capable of reproducing gray scale imagesbecause they are bi-level. As a consequence, binary representation ofgray scale images is a requisite in a wide range of applications such aslaser printers, facsimile machines, lithography (newspaper printing),liquid crystal displays and plasma panels. Gray scale images aretypically converted to binary images utilizing halftone techniques.Halftoning renders the illusion of various shades of gray by using onlytwo levels, black and white, and can be implemented either digitally(facsimile machines, laser printers) or optically (newspaper printing).

Most information display devices are binary in nature, whereas mostimages are continuous in tone. Therefore, the ability to displaycontinuous tone images on binary devices is very useful. However, theproblem of optimally displaying continuous images in a binary formremains unsettled. This problem arises in many forms of media transfer,from graphic arts to facsimile machines. Virtually all printed images inbooks, magazines, newspapers, etc. are composed in a binary nature.Computer hard copy devices are almost exclusively binary in nature. Theprocess of transforming continuous gray scale information into binaryinformation which is perceived to contain a continuous tone, is calledhalftoning.

A continuous image is one that can be defined as “natural”. It is onewhich contains indistinguishable transitions from one gray level to thenext. A binary image is one that is composed of picture elements thatare either black or white. Therefore, the display of a gray scale imageon a binary output device requires that the continuous image bequantized into two levels.

Desirable halftone algorithm characteristics include: implementationsimplicity; reproduction accuracy for both low frequency (or constant)and high frequency (or edges in fine detail); and the absence of visualartifacts such as low frequency Moiré pattern (aliasing) and falsequantization contours (artificial boundaries). Essentially, the desiredresult of the halftoning process is such that the halftone imagesobserved at normal viewing distances of 30-45 centimeters show dotdispersion which is perceived as varying gray levels, while theunderlying dot structure remains unnoticed.

Ordered dither is a halftone technique which represents continuous toneswith clusters of dots arranged to give darker or lighter patterns. Inputvalues are compared against a fixed sized screen, and dots are added tothe dither lattice with increasing gray levels. Ordered dithertechniques include white noise, cluster-dot and dispersed-dot. Thedisadvantages of ordered dither algorithms include loss of most finedetail and the introduction of periodic artifacts. See DigitalHalftoning by R. Ulichney, MIT Press, Cambridge, Mass. (1987).

The major ordered dither techniques are the clustered-dot dither anddispersed-dot dither techniques. Stochastic halftoning processes arepossible but will be addressed later. Of the two techniques, clustereddot is by far the most used, since it reproduces well with xerographicand similar electrostatically based printing technologies. Both of thosetechniques are based upon a threshold array pattern that is of a fixedsize. For example, 6×6 threshold arrays may be compared with the digitalinput values. If the input digital value is greater than the arraypattern number, a 1 is produced and, if it is less, a 0 value isassigned. The number of levels that can be represented using eithertechnique depends on the size of the array. For example, a 6×6 array canproduce 36 unique levels. However, the larger the array the lower thescan frequency, and hence the greater the loss in picture detail.

When assessing the quality of a binary xerographic printer, two measuresare important: the halftone frequency (i.e. number of halftone cells perlinear inch), and the number of distinguishable gray steps. To produce acopy of a picture with a just acceptable degree of halftone graininessrequires at least 65 halftone cells per inch measured along a diagonalof the page. Good quality halftones require about 100 cells/inch, whilehigh quality magazines typically use 150 cells/inch or higher. Theneeded number of distinct gray steps in the pictorial copy depends uponthe eye's ability to distinguish closely spaced grays. A rule of thumbin the printing industry is that an acceptable picture should containabout 65 gray steps. For good quality, 100 or more steps are desired.However, in a binary printer, the maximum number of output gray steps islimited to the number of pixels per halftone cell (p), plus 1. Thus fora typical 8 by 4 rectangular halftone cell, p+1=33 output gray steps.Higher halftone frequencies have fewer pixels per cell and thereforeproduce fewer gray steps. This is the fundamental limitation of binaryprinters.

More levels can be achieved with larger patterns, however, a reductionin the effective resolution occurs because the ability to transitionamong levels is at a coarser pitch. At the pixel rate of about 300 to600 per inch, which is the average pixel rate of copiers and laserprinters, the pattern artifacts are visible for screen patterns largerthan 4×4, and, since 16 levels do not provide an adequate precision fortypical continuous-tone imagery, a suboptimal resolution is usuallyobtained.

Line screening is another halftoning technique. Utilizing a rasteroutput scanning (ROS) approach in combination with pulse widthmodulation (PWM) techniques, line screens enjoy good detail resolutionand freedom from moiré problems. This is particularly so when extendedinto high addressability (HA) by use of pulse width position modulationtechniques (PWPM). High addressability is characteristic of a systemwith sub-pixel addressability. This is achieved with PWPM by using alaser with a spot size significantly smaller than a pixel, and by usingpositioning circuitry capable of starting and stopping the laser pulseat sub-increments of a pixel. This further increases the detailresolution of the system.

However, there are still difficulties with line screens and PWPM, inrendering faithful or pleasing copies of continuous tone originals. Theusual discharge characteristic of the photoconductor and solid areadevelopability of the xerographic development system combine to yield aTone Reproduction Curve (TRC) with a steep slope and a narrow range. Atone end of the gray scale spectrum there is a finite limit in thesmallest amount of charge that can be developed and attendantlimitations in the minimum amount of toner which can be expected toadhere to that charge. At the other end of the gray scale spectrum thereis a point at which the volume of toner developed swamps out the smallundeveloped areas. The result is a copy with washed out highlights andoverdeveloped shadows.

One technique applied to this problem is taught in U.S. Pat. No.5,587,772 to Arai et al. Disclosed here is a pulse width modulationsystem where the clock frequency for a line type halftoner is toggledbetween two frequencies, utilizing lower frequency line types toovercome cost and provide improved natural image quality withoutsacrificing character image quality. A discrimination device determineswhether an image density signal belongs to a line image or to a naturalimage portion of an image. The discrimination device provides a signalto select from two image density conversion devices. The first imagedensity conversion device having a image conversion property forapplication to most of the image density range except the low densityportion. The second image density conversion device having a secondconversion property different from the first and for converting a rangeof image density signal corresponding to a low density portion. A pulsewidth modulation device modulates the conversion signals output from thefirst and second image density conversion devices and provides a pulsewidth modulation signal. A pulse width modulation signal period changingdevice is provided for changing the period of the pulse width modulationdevice output from the pulse width modulator and the period of selectionin accordance with the discrimination signal output from the imagedensity signal discrimination device. The main purpose of the apparatusis to realize line-reduction owing to the property of one of theconversion means that the conversion means outputs a value within arange corresponding to portions where the electrostatic latent image isnot developed.

Stochastic screen is yet another halftoning technique. A stochastichalftone cell is a large threshold array that produces random appearingpatterns in the halftone image. One of the advantages of stochastic, ornon-periodic screening over periodic screening is the suppression ofmoiré. However it has a less desirable image quality, having higherimage noise which leads to more grainy looking images than for example,clustered dot. U.S. Pat. No. 5,673,121 discloses an idealized stochasticscreen, characterized by all of the predominant color dots (black orwhite) being uniformly distributed. The invention seeks to approach thisoptimization by iteratively selecting pairs of threshold levels in thescreen matrix, and measuring the approach to the idealized stochasticscreen. The threshold values are then swapped in position to determinewhether the swap improves the measurement or not. If it does, the swapis maintained. The process is iterated until the desired result isobtained. It should be noted that stochastic screens are very desirablein implementing digital watermarks as taught in U.S. Pat. No. 5,734,752to Knox, and incorporated by reference herein.

In U.S. Pat. No. 5,394,252 to Holladay et al., discloses an imageprocessing system for preparing a color document for printing. Whereeach discrete area or pixel in the image is described by a signal havinga number of possible states or color separations. Each separation of theimage is halftoned, with at least one of the separations processed witha non-periodic halftoning method, and at least one of the remainingseparations processed with a periodic pattern. Preferably, this isperformed in a printer printing with colorants approximating cyan,magenta, yellow and black, where one of the non-yellow separations isprocessed with the non-periodic halftoning method. The teaching here isthe avoidance of moiré and color degradation due to spatial shifting byapplying different halftone techniques to different separations.Advantage is made of utilizing different halftone techniques upondifferent separations where one image separation is printed over topanother. However, no solution is provided to address the need forapplying different halftone types to different segments or density areaswithin a single separation or image.

As may be noted from the discussion above, while there are many halftonetechniques, they all posses strengths and weaknesses. Therefore, itwould be desirable to have a method and apparatus which would allow thecombination of different given halftone techniques with their attendantstrengths to be applied to correspondingly appropriate segments withinan image.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method to enablemaximum picture contrast, highlight, shadow, and color fidelity whileretaining the maximum picture detail in a digital image.

In accordance with the invention, there is provided method and apparatusto enable hybrid screening with the mixing of multiple halftone typesincluding though not limited to stochastic, line screen, and clustereddot types, any or all within a single digital image.

In accordance with the invention, there is provided method and apparatusto enable the maximum image detail resolution but allow the inclusion ofdigital watermarking.

More particularly, the present invention relates to a combination of aselector circuit and high addressability halftone converter circuits.Each halftone converter circuit implements a distinct type of halftoneconversion. The halftone conversion is performed on a pixel by pixelbasis. The selector circuit determines which halftone conversion is tobe used for a given pixel of data. In one embodiment the selectorcircuit determination is dependent upon the given pixel data value. Inan another embodiment the determination results from the output of asegmentation algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a Raster Output Scanner (ROS),illustrating a portion of the photosensitive image plane;

FIG. 2 is a schematic illustration of a preferred embodiment for theinvention;

FIG. 3 is a portion of the photosensitive image plane schematicallydepicting a transition in image density passing through a threshold andthe resulting change in halftone type.

DESCRIPTION OF THE INVENTION

For a general understanding of the present invention, reference is madeto the drawings. In the drawings, like reference numerals have been usedthroughout to designate identical elements. FIG. 1 shows a raster outputscanner (ROS) which may be used to print video signals produced by asource (not shown). There are two common types of ROS 18, flying spotand pulse imaging ROSs. In both, a laser beam 20, emitted from laser 22,passes into conditioning optics 24 which includes a modulator 25. Forprecise periods of time, determined in response to video signalssupplied to ROS 18, modulator 25 either blocks or deflects the laserbeam, or allows to the beam to pass through the conditioning optics toilluminate a facet 26 of rotating polygon 28. Laser 22 may be ahelium-neon laser or a laser diode. In the latter case, the video datawould directly modulate the laser rather than modulator 25. In addition,more than a single laser source 22 or beam 20 could be employed topractice the invention.

After reflecting off facet 26, laser beam 20 passes through conditioningoptics 30 and forms a spot 21 on photosensitive image plane 32. Therotating facet causes laser spot 21 to scan across the image plane in aline 34. Line 34 lies in what is commonly referred to as the fast scandirection, represented by arrow 36. In addition, as facet 26 rotates,image plane 32 moves in a slow scan direction, substantiallyperpendicular to the fast scan direction, as represented by arrow 38.Movement in the slow scan direction is such that successive rotatingfacets of the polygon for successive scanlines 34 that are offset fromeach other in the slow scan direction.

Each scan line 34 includes a row of pixels 40, wherein the pixels areproduced by the modulation of the laser beam as laser spot 21 scansacross the image plane. As beam 20 scans across the scanline, pixel spot21 either illuminates or does not illuminate the individual pixel, inaccordance with the video signals provided to ROS. In general, the videosignals may be characterized as a serial stream of binary pulses, wherea logic one or a pulse specifies that the beam is to illuminate thesurface, while a logic zero, no pulse, will result in no illumination.

For both types of ROS, the width of pixel 40 is dependent upon theperiod or duration of the corresponding logic one pulse in the videosignal supplied to ROS 18. In a scanning spot ROS, at the leading edgeof a pulse modulator 25 allows the passage of laser beam 20 onto theimage plane. For the duration of the pulse, and oval shaped laser spot21 is scanned across image plane 32, illuminating at least one addressedpixel 40 within the scan line 34. The width of the illuminated region inthe fast scan direction thus depends on the duration of the video pulse,as well as on the width and scanning rate of laser spot 21. Typically,the dimensions of the laser spot are such that it is two to three timestaller in the slow scan direction than its width in the fast scandirection. As an example, in a 600 spi, 135 ppm, dual beam printer, thelaser spot is approximately 43 μm high and 20 μm wide, and the timeperiod required for the spot to scan across the width of a single pixel40 is about 15 nanoseconds.

Typically, the video data used to drive the ROS is clocked so that theperiod within which each pixel is exposed, referred to hereafter as apixel clock period, is the same. In addition, the video data used togenerate the video signal pulses which drive the modulator are alsosynchronized with ROS 18 and the movement of the image plane 32 in theslow scan direction, thereby allowing a particular bit of video data toaddress an appropriate portion of image plane 32. The synchronization ofthe video data, the video signal pulses produced therefrom, the ROS andthe image plane is achieved through the use of a system clock that isequivalent to the rate at which pixels must be exposed on the imageplane. Without more, this arrangement limits the addressability for aparticular bit of video data to a selected pixel location. This isreferred to as the inherent addressability. The system clock frequencyis the same as the effective pixel frequency. Using a system clockrunning at a higher frequency than the effective pixel frequency allowssub-pixel addressing. Such an ability when present is referred to asHigh Addressability (HA). While faster clocks may allow greaterresolution within the video pulse stream, a higher frequency alsoresults in increased costs for faster hardware within the videoprocessing path. A technique to allow HA without the need for fasterhardware is pulse width position modulation.

In the following embodiment, a pulse width and position modulator (PWPM)is utilized to form the video signal pulses in response to video datarepresenting the image to be printed. PWPM techniques are well known inthe art. Exemplary examples of which are provided in U.S. Pat. No.5,184,226 and U.S. Pat. No. 5,504,462 both incorporated by referenceherein. The present invention is directed to enabling a PWPM System toprovide a variety of halftone types within a single image whileresponsive to the video data. In this manner the best attributes ofdifferent halftone types are mixed enabling a digital image thatmaximizes both image detail as well as the highlights, shadows and colorfidelity in an image. It should be noted that while the followingdescription is directed toward a single color output, there is no intentto limit the application of the present invention in such a manner.

Turning now to FIG. 2, depicted is a preferred embodiment of theinvention, a pipeline implementation. Halftone selector 210 receives asinput pixel clock 212, and pixel data 214. Each new byte of pixel data214 is latched into the halftone selector 210 with each toggling of thepixel clock 212. A byte of pixel data 214 is then examined in thehalftone selector 210. In a preferred embodiment, examination of a byteof pixel data 214 involves comparing its numerical value as against twoempirically derived threshold values. Any byte of pixel data 214 with anumerical value below or equal to a first threshold value is assigned aunique tag value. Any byte of pixel data 214 with a numerical valueabove or equal to the second threshold value is assigned the same tagvalue. All other bytes of pixel data 214 with values in-between receivean alternative tag value. In a preferred embodiment the threshold valuesare set to approximately 12.5% and 87.5% of the desired gray scalerange. This allows the lightest and the darkest (or highest and lowestdensity) ranges to be tagged as distinct from a middle gray scale range.

In an alternative preferred embodiment the utilization of segmentationtechniques are employed by the halftone selector 210 in the examinationof pixel data 214. Segmentation techniques in the imaging arts arecommonly known and practiced. Segmentation allows the differentiation oftext as from pictorial information, as well as the identification ofshapes, edges, and other sub-area image data of interest. One example isfound in U.S. Pat. No. 5,327,262 the disclosure of which is incorporatedby reference herein. The teaching contained therein is directed todealing with noise found in images. However, similar techniques can beused to isolate highlight and shadow areas as well.

The appeal in employing segmentation techniques within the presentinvention is to allow the identification of very low and very highdensity sub-areas within a given image. Such image sub-areas will bestrespond to application of lower frequency cluster dot type halftones andbenefit from the improved range of grayscale halftones and color tonefidelity attributable to those types. In this way emphasis is given toareas at the extremes of lightness and darkness for monochromaticimages, or the extremes of color saturation for chromatic images. Aunique tag value is assigned to the pixel data 214 found in such imagesub-areas as identified by the segmenter, with an alternate tag valueassigned for all remaining pixel data 214.

Further alternatives, as apparent to one skilled in the art may beemployed in the operation of halftone selector 210. As yet anotherexample, tag generation may be driven by graphic software where aparticular halftone type is elected so that a desired artistic or visualeffect may be realized. Never-the-less, in any version of halftoneselector 210, including the threshold type or the segmenter type, anappropriate tag 216 is generated for each byte of pixel data 214.

On the next toggle of pixel clock 212, the pixel data 214 and tag 216are made available and latched into the halftoner and controller 220. Ina preferred embodiment, the halftoner is a high addressability clustereddot type of halftoner. The controller functions include data remapping,pulse position and inversion signal generation.

In one example, halftoner and controller 220 when presented with a tag216 indicating the pixel data as being found between the upper and lowerthreshold values, will line screen the associated pixel. Line screeningis performed by alternating the position pulse 222, fully leftjustified, then fully right justified, then left again, etc. and feedingthe pixel data into the data lines of the PWPM device 230. Where thepixel data value translates directly into the pulse width, the toggledposition pulse 222 sets the video pulse position to either full left orfull right justification. This creates a line screen with a frequency ofone half of the Image Output Terminal (IOT) spots per inch or one halfthe pixel clock rate. This would mean for example that a 300 linefrequency line screen can be generated from 600 spot per inch pixeldata. In operation this means that the pixel data 214 is passedunmodified directly into the PWPM 230 but coupled with a generatedtoggled position pulse 222. In a further example, an immediatelysubsequent pixel of data 214 with the same tag 216 value would invokenearly the same halftoner and controller 220 response. Once again thepixel data 214 is passed unmodified directly into the PWPM 230 alongwith a toggled position pulse 222. However, the toggle position pulse222 would now indicate the alternative position value and thus thejustification applied to video pulse 232 shifts from fully rightjustified to fully left justified, or from left to right as the case maybe, alternating from whatever its prior justification was.

In the alternative example, halftoner and controller 220 is presentedwith a tag 216 indicating the pixel data as being found below or abovethe upper and lower threshold values, and therefore a differentgrayscale halftone technique is applied to the pixel data 214 in a manordistinct from the above line screen technique. There are many differentgrayscale techniques which may be applied. Table 1 shows one example ofhow a number of various grayscale techniques may be rendered operableutilizing pulse width modulation techniques and in particular PWPM.

TABLE 1 High Addressability Mapping to PWPM HA halftoner RemappedPosition Pixel output Invert Pixel Data Data Data PWPM Video PulseSignal (Decimal) (Binary)  0(0000) 0 0 XX  1(0001) R¼ 64 11  2(0010) C¼64 00  3(0011) R½ 128 11  4(0100) C¼ 64 00  5(0101) NA (R½) 128 11 6(0110) C½ 128 00  7(0111) R¾ 192 11  8(1000) L¼ 64 10  9(1001) C½ X128 00 10(1010) NA (L½) 128 10 11(1011) C¼ X 64 00 12(1100) L½ 128 1013(1101) C¼ X 64 00 14(1110) L¾ 192 10 15(1111) 1 255 XX

For the purpose of discussion, the examples of allowable pixel dataoutput from a halftoner as shown in Table 1 are limited to a four bitrange of 16 possible combinations. These run from zero, no video pulse,to fifteen which equates to a video pulse width which is on for anentire pixel clock period. The first column of Table 1 lists the sixteenpossible combinations. Because here these combinations manipulate pulsewidths which are one quarter of the pixel clock period and address theminto any quarter pixel, they are considered high addressability halftonedata.

The second column of table 1 shows the PWPM interpretation given to thehalftoner output listed in column one. A PWPM can only normally output asingle pulse, consisting of first a rising edge followed by a fallingedge. The width of the pulse may be ¼, ½, ¾, or the full width of thepixel in the Table 1 example. These pulse widths correspond to pixeldata values of 64, 128, 192 and 255 respectively (expressed in decimalnotation) as remapped for input into the PWPM. This is shown in columnfour of Table 1. The ¼, ½, ¾pulse examples may be left (L), right (R),or center (C), justified as determined by the two bit position data aslisted in column five of Table 1. A PWPM is normally limited togenerating a rising pulse edge followed by a falling pulse edge. Sothere are some combinations which are not possible for the PWPM toprovide. Combination examples 9, 11, and 13, may never-the-less beachieved by toggling a selectable analog inverter circuit as provided onthe output of the PWPM when used in conjunction with the remapped pixeland positional data as shown in the Table 1. Finally there are two pulsewidth, position combinations listed in the table which the PWPM cannotdo, examples 5 and 10. That is because these combinations require atleast three pulse edges, a thing which a conventional PWPM cannot do. Itis unlikely that such a combination would be required from customaryhalftoners, but if needed the combinations may be approximated by thePWPM pulse as parenthetically shown in Table 1.

Returning now to FIG. 2 and our alternative example, where the halftonerand controller 220 is presented with a tag 216 indicating a differentgrayscale halftone technique is to be applied to incoming pixel data214. The halftoner portion of the halftoner and controller 220 must nowgrayscale the pixel data 214 into halftone data instead of simplypassing it on. In a preferred embodiment, the grayscale techniqueapplied to the pixel data 214 is a cluster dot halftone. Cluster dothalftone grayscaling is common and well understood in the imaging arts.Exemplary examples of which are found in U.S. Pat. Nos. 4,149,194, and4,185,304 both of which are incorporated by reference herein. The graylevel value of a pixel of data is compared against a threshold value asfound in a threshold array stored in memory. Depending upon the result,the appropriate halftone data is provided. In an preferred embodimentthis is a four bit nibble as listed in the first column of Table 1. Thisfour bit nibble is passed on to the controller section of halftoner andcontroller 220. The controller remaps the four bit nibble into theappropriate PWPM 230 input data 221 and position data 222 as shown incolumns four and five of Table 1 respectively. In addition, as neededthe controller section of halftoner and controller 220 will generateinvert signal 224 for an analog inverter circuit 240.

With either of the above examples of tag 216 toggle position, the PWPM230 latches in data 221 and position data 222 on the next clock pulse212. The PWPM then generates video data 232 which is sent to inverter240. The output of analog inverter 240 is final video pulse 242.

FIG. 3 depicts the result of applying a preferred embodiment approach asimage density varies on image plane 32. The arrow 36 represents the fastscan direction and arrow 38 represents the slow scan direction. Amultiplicity of pixel 40 are arranged in rows and thereby compose scanlines 34. Tic marks 341 are provided as indicia of the pixel grid andaid in delineating the row and column arrangement of pixels. In apreferred embodiment a transition of halftone type occurs atapproximately the 12.5% image density point. As an aid to understanding,indicia of the transition point is provided here by line 342. Thehalftone data above line 342 is rendered utilizing line screentechniques. The halftone data found below line 342 is rendered utilizinga preferred embodiment clustered dot technique.

Dotted lines are provided to indicate a preferred embodiment clustereddot halftone cell 344. The halftone cell 344 is a matrix four pixels 40wide and six pixels 40 high. The clustering starts in the center of thefirst row and builds up row by additional row. This yields a six stepgradation in tone between zero density and a preferred embodiment 12.5%density threshold. The cluster dot result 346 is an example of theminimum density and size dot for this embodiment. Cluster dot result 346is an indication of image density running at approximately 2%. This dotis built using Table 1 examples 1 and 8 to derive right and leftjustified quarter width PWPM pulses R¼ and L¼. The R¼ pulse is placed inthe 1st row, 2nd pixel location, and the L¼ pulse is placed in the 1strow, 3rd pixel location. This effectively yields a centered half widthpulse as shown in FIG. 3, cluster dot result 346. Incremental increasesin image density are further provided with cluster dot results 348, 350,352, 354, and finally 356 where a density of approximately 12.5% isreached. While the cluster dot technique employed here is capable ofdensities greater than this, in a preferred embodiment such as this,higher densities will be rendered with a line screen halftone techniqueas directed by the halftone selector 210. Thus a wider cluster dotresult will not be applied. However, note that cluster dot result 350 byvirtue of starting immediately where cluster dot result 356 has finishedcreates in effect a larger apparent dot.

The advantage of utilizing the clustered dot halftone techniquedescribed above is in the improved rendering of image highlight andshadow. This is achieved by relieving the system of the need to developvery small areas and hold very small amounts of toner together on aphotosensitive image plane 32 (or the inverse at the opposite end of thedensity spectrum—development which leaves very small areas undevelopedwith very small areas without toner). The area of toner is larger, butthe ratio of toner to non-toner area is smaller. The eye will blend theratio of areas and perceive a lightening of tone density. This is usefulin image segments with little detail or only gradual sifts in grayscale. It is also particularly useful in color systems where the eyewill blend the small areas of color toner together, much as the Frenchpainter Seurat demonstrated with his pointillistic artwork. In thismanner the system will appear to be yielding better color fidelitybecause the intended ratio amongst the primary colors is betterpreserved.

Differing advantages may be realized by utilizing different halftonetechniques as would be appreciated by one skilled in the art. Forexample as a further preferred embodiment, a stochastic halftonetechnique may be employed to yield two possible improvements. Stochasticscreens have been shown of value in digital watermarking. With thepresent invention, a line screen approach could be maintained in areasof image detail while with a change in halftone, other shaded areas mayreceive a watermark. Secondly, stochastic halftoning is attributed inblack and white conversions of color images as providing a better senseof fidelity with the original. This may also be of particular value ingraphic design systems with the rendering of mechanical and other typesof generated drawings. All of the above and other alternativecombinations and variations suggested by the teaching contained hereinare considered as encompassed in the present invention.

While the embodiment disclosed herein is preferred, it will beappreciated from this teaching that various alternative, modifications,variations or improvements therein may be made by those skilled in theart, which are intended to be encompassed by the following claims:

What is claimed is:
 1. An apparatus for halftone hybrid screengeneration, comprising: a halftone selector, receiving video signalscorresponding to gray scale pixel data and supplying a tag valueresponsive of an attribute identified with the incoming pixel data; ahalftoner, receiving the gray scale pixel data and the tag value fromthe halftone selector, converting the gray scale pixel data intohalftone data and responsive to the tag value passing either the grayscale pixel data or the halftone data; a pulse width modulator,receiving the data from the halftoner and generating a video pulse foran image output terminal.
 2. The apparatus for halftone hybrid screengeneration of claim 1, wherein the pulse width modulator is a pulsewidth position modulator.
 3. The apparatus for halftone hybrid screengeneration of claim 1, wherein the halftone selector is a segmentertype.
 4. The apparatus for halftone hybrid screen generation of claim 2,wherein the pulse width position modulator produces line screenhalftones.
 5. The apparatus for halftone hybrid screen generation ofclaim 4, wherein the halftoner produces clustered dot halftones.
 6. Theapparatus for halftone hybrid screen generation of claim 4, wherein thehalftoner produces high addressability clustered dot halftones.
 7. Theapparatus for halftone hybrid screen generation of claim 1, wherein thehalftone selector is a threshold comparison type.